<template>
  <ul class="list">
    <li class="item" v-for="item in letters" :key="item" :ref="item"
        @click="handleLetterClick" @touchstart="handleTouchStart" @touchmove="handleTouchMove"
        @touchEnd="handleTouchEnd">{{item}}
    </li>
  </ul>
</template>

<script>
  export default {
    name: "CityAlphabet",
    data() {
      return {
        touchStatus: false,
        startY: 0,
        timer: null,
      }
    },
    updated() {
      this.startY = this.$refs['A'][0].offsetTop;
    },
    props: {
      cities: Object,
    },
    computed: {
      letters() {
        const letters = [];
        for (let i in this.cities) {
          letters.push(i)
        }
        return letters
      }
    },
    methods: {
      handleLetterClick(e) {
        this.$emit('change', e.target.innerText)
      },
      handleTouchStart() {
        this.touchStatus = true
      },
      handleTouchMove(e) {
        if (this.touchStatus) {
          if (this.timer) {
            clearTimeout(this.timer)
          }
          this.timer = setTimeout(() => {
            const touchY = e.touches[0].clientY - 89;
            const index = Math.floor((touchY - this.startY) / 20);
            if (index >= 0 && index < this.letters.length) {
              this.$emit('change', this.letters[index])
            }
          }, 8) // 限制handleTouchMove的执行频率, 优化性能 (8ms)
        }
      },
      handleTouchEnd() {
        this.touchStatus = false;
      },
    }
  }
</script>

<style lang="stylus" scoped>
  @import "~styles/varibles.styl"
  .list
    display flex
    flex-direction column
    justify-content center
    position absolute
    top 1.58rem
    right 0
    bottom 0
    width .4rem

    .item
      text-align center
      line-height .4rem
      color $bgColor

</style>
